It is the function of dialysis equipment to detoxicate the blood of uremic patients through a semipermeable membrane. A dialyzing fluid is passed through one chamber of the dialyzer and the blood to be detoxicated is passed through another chamber along the other side of the membrane. Waste matter passes from the blood through the membrane because of the lower concentration of waste matter in the dialysate solution than in the blood.
In addition to the detoxication of the blood, it is necessary to withdraw excess water from the blood of the patient. Removal of water from the blood is known as ultrafiltration. Ultrafiltration is accomplished by applying a pressure difference across the membrane (transmembrane pressure) so that the blood-carrying side of the membrane has a positive pressure and the side carrying dialyzing fluid has a negative pressure or a pressure which is less positive. The rate of ultrafiltration is dependent upon the transmembrane pressure. The higher the transmembrane pressure, the greater is the removal of water from the blood.
Because of the wide difference in the ultrafiltration output among various dialyzers, it is necessary to monitor the withdrawal of fluid, e.g. by periodically weighing a patient or by using a bed scale. An uncertainty in the ultrafiltration process is the preselection of the transmembrane pressure required for obtaining the desired ultrafiltration rate. It would be better, instead of preselecting the transmembrane pressure, [i.e., in the usual practice preselecting the negative pressure of the dialysis liquid] to be able to set or indicate directly the desired ultrafiltration rate. In principle, indication of the quantity of water removed by ultrafiltration is possible by the use of precision flowmeters. However, despite extremely accurate measuring equipment, measurement is only possible within very limited accuracy because the quantity of water removed by ultrafiltration is very small in proportion to the quantity of dialysate.
A known dialysis system is disclosed in U.S. Pat. No. 3,844,940. This prior art dialysis system comprises a switching device for disconnecting the dialysate path containing one chamber of the dialyzer from the dialysate source. A controlled suction pump positioned downstream from the dialyzer maintains the transmembrane pressure constant during an interruption phase. The quantity of liquid transported out of the dialyzer substantially corresponds to the quantity of liquid removed by ultrafiltration. The liquid is conveyed into a measuring vessel which comprises a level gauge having two capacitor electrodes. In this manner, the quantity of liquid removed from the blood by ultrafiltration during the disconnection of the dialysate source from the dialysate path is measured.
The measuring phase is the phase when the rate of ultrafiltration or the quantity of liquid removed from the blood by ultrafiltration is measured. During the measuring phase, the dialysate source has been disconnected.
The operative phase is the phase when the dialysate source is connected to the dialysate path and providing dialysate solution for flow through one chamber of the dialyzer. During the operative phase, detoxification is taking place. Ultrafiltration is also taking place dependent upon the magnitude of the transmembrane pressure.
The known dialysis apparatus has the disadvantage that only the blood pressure is measured for the determination of the transmembrane pressure. This blood pressure value is compared with the pressure at the exit from the dialysate chamber and the difference is used as the transmembrane pressure. Since the dialysate does not flow through the dialyzer during the interruption phase, the flow resistance or respectively the pressure drop in the dialyzer is not taken into account. In addition, the venous blood pressure measured on the patient does not furnish a representative value for the pressure in the blood chamber of the dialyzer. During the measuring phase, therefore, pressure conditions prevail in the dialyzer which differ considerably from the pressure conditions during the operative phase. The device for measuring the transmembrane pressure regulates the suction pump during the measuring phase in a manner which does not correspond to the conditions during the operative phase. In addition, the known system involves merely a measuring system which does not indicate how the transmembrane pressure must be changed in order to obtain a desired ultrafiltration flow. Furthermore, the liquid volume measuring device, operating with two electrodes, is relatively imprecise because its result depends on the conductivity of the dialysate solution.